Downhole pump assembly and method of recovering well fluids

ABSTRACT

The present invention relates to a downhole tool. In particular, the present invention relates to a downhole pump assembly, a downhole tool assembly including a downhole pump assembly, a well including a downhole pump assembly and to a method of recovering well fluids. In one embodiment of the invention, there is disclosed a downhole tool assembly ( 10 ) for location in a borehole ( 16 ) of a well ( 12 ), the tool assembly ( 10 ) including a downhole pump assembly ( 18 ). The pump assembly ( 18 ) comprises a turbine ( 26 ) coupled to a pump ( 28 ), for driving the pump ( 28 ) to recover well fluid.

BACKGROUND OF THE INVENTION

The present invention relates to a downhole tool. In particular, thoughnot exclusively, the present invention relates to a downhole pumpassembly, a downhole tool assembly including a downhole pump assembly, awell including a downhole pump assembly and to a method of recoveringwell fluids.

FIELD OF INVENTION

In the field of oil and gas well drilling, it is sometimes necessary toemploy “artificial lift” techniques to recover reservoir fluids from awell borehole. Currently this may be achieved by using an electricalsubmersible pump (ESP), which includes a pump driven by an electricmotor, which is run into the borehole to recover reservoir fluids tosurface through the borehole. The ESP includes power and control cablesextending from the surface and electrical connections in the downholeenvironment. This causes significant problems, in particular becausetypical reservoir depths may be between 1,000 to 10,000 ft, and thecables must be trailed over this length to surface. Also, the electricmotor, power cable and electrical connections are typically associatedwith the highest causes of failure in ESP's. Further equipment includinga downhole isolation chamber, surface switchboard and surface powertransformer must also be provided. Typical ESP's also include insulationsystems and elastomeric components, which are adversely effected by theextreme pressures and temperatures experienced downhole. These factorsall contribute to provide significant disadvantages in the use of ESP's,in particular in terms of their running life and maintenance costs.

It is amongst objects of at least one embodiment of at least one aspectof the present invention to obviate or mitigate at least one of theforegoing disadvantages.

SUMMARY OF INVENTION

According to a first aspect of the present invention, there is provideda downhole pump assembly comprising a turbine coupled to a pump, fordriving the pump.

The pump assembly may be for driving the pump to recover well fluid. Thewell fluid is recovered to surface, and may take the form of hydrocarbonbearing reservoir fluid such as oils. Typically, the downhole pumpassembly is for location in a casing/lining in a borehole of a well, andthe pump assembly may be for coupling to downhole tubing for location inthe borehole.

Preferably, at least part of the pump is isolated from at least part orthe turbine. The pump may include a pump fluid inlet and a pump fluidoutlet, and the pump inlet may be fluidly isolated from at least part ofthe turbine. In particular, the pump fluid inlet may be fluidly isolatedfrom a fluid outlet of the turbine. In this fashion, the pump may beactivated to pump and thus recover mainly well fluid. However, turbinedrive fluid (such as water or steam, where the well fluids comprise verythick or viscous oils) may be carried with the well fluid; the pumpfluid outlet may be disposed in fluid communication with the turbineoutlet, for mixing of the well and turbine drive fluids for recovery.Alternatively, the turbine fluid outlet may also be isolated from thepump fluid outlet, and the turbine fluid outlet may be spaced from thepump for discharging turbine drive fluid at a location spaced from thepump. Beneficially, the turbine fluid outlet is located, in use, furtherdownhole than the pump fluid outlet. Advantageously, this allows, inparticular, the turbine drive fluid to be injected into the formation,ideally at a location spaced perhaps hundreds or thousands of feet fromthe pump. This injected fluid helps to maintain formation pressure atacceptable operational levels for recovery of well fluid. This alsoadvantageously isolates the recovered well fluid from turbine drivefluid, limiting the degree of separation otherwise required at surfaceto obtain the well fluid.

The at least part of the pump may be fluidly isolated from the at leastpart of the turbine by a packer or other isolation means. The pump maybe for location in the packer, such that the packer seals a chamber, inparticular an annulus defined between the pump and a borehole in whichthe downhole pump assembly is located, in particular between the pumpassembly and casing/lining in the borehole. The turbine and pump outletsmay be disposed above or upstream, with reference to the direction ofrecovery of well fluid, of the packer or other isolation means, formixing of the well and turbine drive fluids. Alternatively, the pumpassembly may further comprise discharge means in the form of dischargetubing coupled to the pump assembly and defining an outlet forming afluid outlet of the turbine. This may allow turbine drive fluid to bedischarged at the location spaced from the pump. The turbine outletdefined by the discharge means may be isolated from the pump by a packeror other isolation means.

The turbine may be directly coupled to the pump and the turbine and pumpmay be selected according to desired operating characteristics of one ofthe pump or turbine, to balance, in particular, ideal operatingrotational velocities of the turbine and pump. As will be discussedbelow, the turbine may be adjustable to vary the rotational velocity ofthe turbine, for example by varying a size of a nozzle of the turbine,to balance the flow velocity of fluid flowing through the turbine, andthus the rotational velocity of the turbine, to that of the pump.Alternatively, the downhole pump assembly may further comprise gearmeans such as a gear unit coupling the turbine to the pump. The turbineand pump may include respective bearing assemblies such as one or morethrust bearings, for absorbing axial thrust loading generated by theturbine and the pump, respectively.

The downhole pump assembly may include delivery tubing for supplyingdrive fluid to the turbine and may also include return tubing forreturning well fluid and/or turbine drive fluid to surface. The deliveryand return tubing may comprise coil tubing and may be for coupling todownhole tubing such as production tubing extending from surface. Thedelivery and return tubing may be sealed by a packer or other isolationmeans. This may serve to isolate a generally annular chamber definedbetween a borehole in which the downhole pump assembly is located andthe assembly itself and/or downhole tubing, to constrain return flow tosurface to be directed through the return tubing. Alternatively, thedownhole pump assembly may be for coupling directly to downhole tubingfor supplying turbine drive fluid and the assembly may be adapted torecover well fluid through an annulus defined between a borehole and thedownhole pump assembly and/or downhole tubing. Additionally, where thepump assembly further comprises discharge tubing, the tubing may extendthrough the turbine and pump or be coupled to and extend therefrom, to adischarge location spaced from the pump assembly.

According to a second aspect of the present invention, there is provideda downhole tool assembly comprising downhole tubing and a downhole pumpassembly coupled to the downhole tubing for location in a borehole of awell, the pump assembly including a turbine coupled to a pump, fordriving the pump to recover well fluid.

According to a third aspect of the present invention, there is provideda well comprising:

a borehole;

downhole tubing located in the borehole; and

a downhole pump assembly coupled to the downhole tubing and located inthe borehole in a region of a well fluid producing formation, the pumpassembly including a turbine coupled to a pump, for driving the pump torecover well fluid.

The downhole tubing may comprise production tubing extending fromsurface. The downhole pump assembly may be coupled to the productiontubing by delivery tubing for supplying drive fluid to the turbine andreturn tubing for returning well fluid and/or turbine drive fluid tosurface. The delivery and return tubing may comprise coil tubing, whichmay be banded to the production tubing. The downhole pump assembly mayfurther comprise a packer or other isolation means for constrainingreturn fluid flow to be directed through the return tubing. The packermay seal a generally annular chamber defined between the downhole pumpassembly and the borehole, in particular between the turbine deliverytubing and return tubing, and the borehole. The borehole may be linedwith casing/lining in a known fashion.

Alternatively, the downhole tubing, which may comprise productiontubing, may be coupled directly to the downhole pump assembly. In thisfashion, turbine drive fluid may be directed through the productiontubing to the turbine, and return flow of recovered well fluid and/orturbine drive fluid may be directed along an annulus defined between thedownhole tool assembly and the borehole. Additionally, the pump assemblymay further comprise discharge means in the form of discharge tubingcoupled to the pump assembly and defining an outlet forming a fluidoutlet of the turbine.

Further features of the downhole pump assembly are defined withreference to the first aspect of the present invention.

Preferably, the turbine comprises a tubular casing enclosing a chamberhaving rotatably mounted therein a rotor comprising at least one turbinewheel blade array with an annular array of angularly distributed bladesorientated with drive fluid receiving faces thereof facing generallyrearwardly of a forward direction of rotation of the rotor, and agenerally axially extending inner drive fluid passage generally radiallyinwardly of said rotor, said casing having a generally axially extendingouter drive fluid passage, one of said inner and outer drive fluidpassages constituting a drive fluid supply passage and being providedwith at least one outlet nozzle formed and arranged for directing atleast one jet of drive fluid onto said blade drive fluid receiving facesof said at least one blade array as said blades traverse said nozzle forimparting rotary drive to said rotor, the other constituting a drivefluid exhaust passage and being provided with at least one exhaustaperture for exhausting drive fluid from said at least one turbine wheelblade array.

Preferably also, the turbine has a plurality, advantageously, amultiplicity, of said turbine wheel means disposed in an array ofparallel turbine wheels extending longitudinally along the centralrotational axis of the turbine with respective parallel drive fluidsupply jets.

In a particularly preferred embodiment, the turbine comprises a tubularcasing enclosing a chamber having rotatably mounted therein a rotorhaving at least two turbine wheel blade arrays each with an annulararray of angularly distributed blades orientated with drive fluidreceiving faces thereof facing generally rearwardly of a forwarddirection of rotation of the rotor, and a generally axially extendinginner drive fluid passage generally radially inwardly of each saidturbine wheel blade array, said casing having a respective generallyaxially extending outer drive fluid passage associated with each saidturbine wheel blade array, one of said inner and outer drive fluidpassages constituting a drive fluid supply passage and being providedwith at least one outlet nozzle formed and arranged for directing atleast one jet of drive fluid onto said blade drive fluid receiving facesas said blades traverse said at least one nozzle for imparting rotarydrive to said rotor, the other constituting a drive fluid exhaustpassage and being provided with at least one exhaust aperture forexhausting drive fluid from said turbine wheel blade arrays,neighbouring turbine wheel blade arrays being axially spaced apart fromeach other and provided with drive fluid return flow passagestherebetween connecting the exhaust passage of an upstream turbine wheelblade array to the supply passage of a downstream turbine wheel bladearray for serial interconnection of said turbine wheel blade arrays.

Instead of, or in addition to providing a said inner or outer drivefluid passage for exhausting of drive fluid from the chamber, therecould be provided exhaust apertures in axial end wall means of thechamber, though such an arrangement would generally be less preferreddue to the difficulties in manufacture and sealing.

In yet another variant both the drive fluid supply and exhaust passagemeans could be provided in the casing (i.e. radially outwardly of therotor) with drive fluid entering the chamber from the supply passage vianozzle means to impact the turbine blade means and drive them forward,and then exhausting from the chamber via outlet apertures angularlyspaced from the nozzle means in a downstream direction, into the exhaustpassages.

Thus essentially the turbine is of a radial (as opposed to axial) flownature where motive or turbine drive fluid moves between radially (asopposed to axially) spaced apart positions to drive the turbine blademeans. This enables the performance, in terms of torque and powercharacteristics, of the turbine to be readily varied by simply changingthe nozzle size—without at the same time having to redesign and replaceall the turbine blades as is generally the case with conventional axialflow turbines when any changes in fluid velocity and/or fluid densityare made. Thus, for example, reducing the nozzle size will (assumingconstant flow rate) increase the (fluid jet) flow velocity therebyincreasing torque This will also increase the operating speed of theturbine and thereby the power, as well as increasing back pressure.Similarly increasing flow rate while keeping nozzle size constant willalso increase the (fluid jet) flow velocity thereby increasing torque aswell as giving an increase in the operating speed of the turbine andthereby the power and increasing back pressure. Alternatively,increasing the nozzle size while keeping the (fluid jet) flow velocityconstant—by increasing the flow rate, would increase torque and powerwithout increasing the turbine speed or back pressure. If desired,torque can also be increased by increasing the density of the drivefluid (assuming constant fluid flow rate and velocity) which increasesthe flow mass.

It will be appreciated that individual nozzle size can be increasedlongitudinally and/or angularly of the turbine, and that the number ofnozzles for the or each turbine wheel blade array can also be varied.

The turbine blades can also have their axial extent longitudinally ofthe turbine increased so as to increase the parallel mass flow of motivefluid through the or each turbine wheel array, without suffering thesevere losses encountered with conventional multi-stage turbinescomprising axially extending arrays of axially driven serially connectedturbine blade arrays.

Another advantage of the turbine that may be mentioned is thecircumferential fluid velocity distribution over the turbine blades is,due to the generally radial disposition of the said blades,substantially constant and thus very efficient in comparison with anaxial turbine where the velocity distribution varies over the length ofthe blade and thus losses are caused through hydrodynamic miss-match offluid velocity and circumferential blade velocity.

Another important advantage over conventional turbines for down-hole useis that the motors of the present invention are substantially shorterfor a given output power (even when taking into account any gear boxeswhich may be required for a given practical application). Typically aconventional turbine may have a length of the order of 15 to 20 metres,whilst a comparable turbine of the present invention would have a lengthof only 2 to 3 metres for a similar output power. This has veryconsiderable benefits such as reduced manufacturing costs, easierhandling, and, in particular allows a downhole pump assembly of thepresent invention having a low overall length to be provided.

Yet another advantage that may be mentioned is that the relatively highoverall efficiency of the turbine allows the use of smaller size(diameter) turbines than has previously been possible. With conventionaldown-hole turbines, the so-called “slot losses” which occur due to drivefluid leakage between the tips of the turbine blades and the casing dueto the need for a finite clearance therebetween, become proportionatelygreater with reduced turbine diameter. In practice this results in aminimum effective diameter for a conventional turbine of the order ofaround 10 cm. With the increased overall efficiency of the applicant'sturbine it becomes practical significantly to reduce the turbinediameter, possibly as low as 3 cm.

In one, preferred, form of the turbine the outer passage means serves tosupply the drive fluid to the turbine wheel means via nozzle means,preferably formed and arranged so as to project a drive fluid jetgenerally tangentially of the turbine wheel means, and the inner passagemeans serves to exhaust drive fluid from the chamber, with the innerpassage means conveniently being formed in a central portion of therotor. In another form of the turbine the inner passage means is used tosupply the drive fluid to blade means mounted on a generally annularturbine wheel means. In this case the nozzle means are generally formedand arranged to project a drive fluid jet more or less radiallyoutwardly, and the blade means drive fluid receiving face will tend tobe oriented obliquely of a radial direction so as to provide a forwarddriving force component as the jet impinges upon said face.

In principle there could be used just a single nozzle means. Generallythough there is used a plurality of angularly distributed nozzle meanse.g. 2, 3 or 4 at 180°, 120° or 90° intervals, respectively. In thepreferred form of the turbine, the nozzle means are preferably formedand arranged to direct drive fluid substantially tangentially relativeto the blade means path, but may instead be inclined to a greater orlesser extent radially inwardly or outwardly of a tangential directione.g. at an angle from +5° (outwardly) to −20′ (inwardly), preferably 0°to −10°, relative to the tangential direction—corresponding to from 95to 70°, preferably 90 to 80°, relative to a radially inward direction.

As noted above the power of the motor may be increased by increasing themotive fluid energy transfer capacity of the turbine, in parallel—e.g.by having larger cross-sectional area and/or more densely angularlydistributed nozzles. The driven capacity of the turbine may be increasedby inter alia increasing the angular extent of the nozzle means in termsof the size of individual nozzle means around the casing, and/or byincreasing the longitudinal extent of the nozzle means in terms oflongitudinally extended and/or increased numbers of longitudinallydistributed nozzle means. In general though the outlet size ofindividual nozzle means should be restricted relative to that of thedrive fluid supply passage, in generally known and calculable manner, soas to provide a relative high speed jet flow. The jet flow velocity isgenerally around twice the linear velocity of the turbine (at the fluidjet flow receiving blade portion) (see for example standard text bookssuch as “Fundamentals of Fluid Mechanics” by Bruce R Munson et alpublished by John Wiley & Sons Inc). Typically, with a 3.125 inch (8 cm)diameter turbine of the invention there would be used a nozzle diameterof the order of from 0.1 to 0.35 inches (0.25 to 0.89 cm).

The size of the blade means including in particular the longitudinalextent of individual blade means and/or the number of longitudinallydistributed blade means, will generally be matched to that of the nozzlemeans. Preferably the blade means and support therefor are formed andarranged so that the unsupported length of blade means between axiallysuccessive supports is minimised whereby the possibility of deformationof the blade means by the drive fluid jetting there onto is minimised,and in order that the thickness of the blade means walls may beminimised. The number of angularly distributed individual blade meansmay also be varied, though the main effect of an increased number is inrelation to smoothing the driving force provided by the turbine.Preferably there is used a multiplicity of more or less closely spacedangularly distributed blade means, conveniently at least 6 or 8,advantageously at least 9 or 12 angularly distributed blade means, forexample from 12 to 24, conveniently from 15 to 21, angularly distributedblade means.

It will also be appreciated that various forms of blade means may beused. Thus there may be used more or less planar blade means. Preferablythough there is used a blade means having a concave drive fluidreceiving face, such a blade means being conveniently referred tohereinafter as a bucket means. The bucket means may have various formsof profile, and may have open sides (at each longitudinal end thereof).Conveniently the buckets are of generally part cylindrical channelsection profile (which may be formed from cylindrical tubing section).Optimally, however, the bucket should beaerodynamically/hydrodynamically shaped to prevent detachment of theboundary layer and to produce a less turbulent flow through is theturbine blade array and thus reduce parasitic pressure drop across theblade array.

Various forms of blade support means may be used. Thus, for example, thesupport means may be in the form of a generally annular structure withlongitudinally spaced apart portions between which the blade meansextend. Alternatively there may be used a central support member,conveniently in the form of a tube providing the inner drive fluidpassage means, with exhaust apertures therein through which used drivefluid from the chamber is exhausted, the central support member havingradially outwardly projecting and axially spaced apart flanges orfingers across which the blade means are supported. Alternatively theblade means may have root portions connected directly to the centralsupport member.

The turbine may typically have normal running speeds of the order of,for example, from 2000 to 5,000 rpm. However, small pumps may require torun at higher speeds. Whilst the turbine is preferably directly coupledto the pump, the turbine may alternatively be used with gear box means,in order to increase torque. In this case and in general there may beused gear box means providing around, for example, 2:1 or 3:1 speedreduction. There may be used an epicyclic gear box with typically 3 or 4planet wheels mounted in a rotating cage support used to provide anoutput drive in the same sense as the input drive to the sun wheel,usually-clockwise, so that the output drive is also clockwise. There maybe used a ruggedized gear box means with a substantially sealed boundarylubrication system, advantageously with a pressure equalisation systemfor minimizing ingress of drilling fluid or other material from theborehole into the gear box interior.

According to a fourth aspect of the present invention, there is provideda method of recovering well fluids, the method comprising the steps of:

coupling a turbine to a pump to form a downhole pump assembly;

coupling the pump assembly to downhole tubing;

running the downhole tubing and downhole pump assembly into a boreholeof a well and locating the pump assembly in a region of a well fluidproducing formation; and

supplying drive fluid downhole to drive the turbine, to in turn drivethe pump and recover well fluid from the borehole.

The method may further comprise coupling the pump assembly to productiontubing, and may in particular comprise coupling the turbine to theproduction tubing by turbine delivery fluid tubing, and by return fluidtubing for recovering well fluid and/or turbine drive fluid. The methodmay further comprise supplying drive fluid through the turbine drivefluid delivery tubing to drive the turbine and in turn drive the pump torecover well fluid through the return tubing. The turbine drive fluiddelivery tubing and return fluid tubing may be sealed with respect tothe borehole by isolation means such as a packer. This mayadvantageously constrain well fluid and/or turbine drive fluid to bereturned through the return tubing.

Alternatively, the method may further comprise coupling the pumpassembly, in particular the turbine, directly to production tubing andsupplying drive fluid through the production tubing to drive theturbine. Well fluid may be recovered through an annulus defined betweenthe downhole pump assembly and/or downhole tubing and the borehole.

The method may further comprise isolating an inlet of the pump from anoutlet of the turbine, to isolate the pump inlet from turbine drivefluid. The pump inlet may be isolated from the turbine outlet bylocating isolation means such as a packer around part of the pumpassembly, in particular the pump.

The method may further comprise mixing well fluid with turbine drivefluid discharged from the turbine and returning the well fluid tosurface. The well fluid and discharged turbine drive fluid may be mixedat or in the region of an outlet of the pump. Advantageously, thisisolates the pump inlet such that the work carried out by the pump islargely to pump well fluids to surface. Alternatively, or additionally,the method may further comprise injecting or discharging spent turbinedrive fluid into the formation. This assists in maintaining formationpressure at acceptable levels. This may be achieved by couplingdischarge means to the pump assembly, the discharge means defining aturbine outlet, and by isolating the discharge means outlet from thepump, to direct spent drive fluid into the formation. Preferably, thespent turbine drive fluid is injected at a location spaced from the pumpassembly; typically this may be hundreds or thousands of feet, to avoidthe spent drive fluid being drawn back out of the formation by the pump.

The turbine may be driven at least in part by recovered well fluid.Preferably, the recovered well fluid is separated into at least waterand hydrocarbon components including oils, gases and/or condensates.Separated water, oil or a combination of the two may be used as theturbine drive fluid. Alternatively, the turbine may be driven at leastin part by a gas, such as air or Nitrogen, steam or a foam such asNitrogen foam. It will be understood that, where the turbine is drivenat least in part by recovered well fluid, it may be necessary, at leastinitially, to supply a non-well fluid such as seawater or a mud to theturbine and that following well fluid production or increase in wellfluid production using the pump assembly, recovered well fluid may beused to drive the turbine.

However, it will also be understood that recovered well fluid may beused to dive the turbine from start-up where there is a sufficient flowof well fluids to begin with.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic sectional view of a well comprising a downholetool assembly having a downhole pump assembly, in accordance with anembodiment of the present invention;

FIG. 2 is a schematic sectional view of a well comprising a downholetool assembly having a downhole pump assembly, in accordance with analternative embodiment of the present invention;

FIG. 2A is a schematic sectional view of a well comprising a downholetool assembly having a pump assembly, in accordance with a furtheralternative embodiment of the present invention;

FIG. 3 is an enlarged, detailed view of a turbine power unit formingpart of the downhole pump assemblies of FIGS. 1, 2 and 2A, but withbearing and seal details omitted for greater clarity;

FIG. 4A is a transverse section of the turbine unit of FIG. 3, takenalong line 4A-4A;

FIG. 4B is a detailed view showing part of a downhole pump assemblysimilar to that shown in FIGS. 1 and 2, but including a turbine havingupper and lower turbine units similar to that shown in FIG. 3, FIG. 4Bbeing a detailed view showing the connection between the upper and lowerturbine units;

FIG. 5 is a partly sectioned side elevation of the main part of theturbine rotor of FIGS. 3 and 4B without bucket means;

FIG. 6 is a transverse section of the rotor of FIG. 5 with bucket meansin place, taken along line 6-6;

FIG. 7 is a transverse section of the rotor of FIG. 5 with bucket meansin place, taken along line 7-7;

FIG. 8 is a transverse section of an epicyclic gear system, coupled tothe turbine of FIG. 3/4B and forming part of a downhole pump assembly inaccordance with a further alternative embodiment of the presentinvention;

FIGS. 9-13 show an alternative turbine forming part of the downhole pumpassemblies shown in FIGS. 1 and 2 in which:

FIG. 9 is a longitudinal sectional view corresponding generally to thatof FIG. 3;

FIG. 10 is a transverse section taken along line 10-10 of FIG. 9;

FIG. 11 is a transverse section taken along line 11-11 of FIG. 9;

FIG. 12 is a perspective view showing the principal parts of the turbineof FIGS. 9-11 with the outer casing removed; and

FIG. 13 is a view corresponding to FIG. 12 but with part of the statorremoved to reveal the rotor.

DETAILED DESCRIPTION OF DRAWINGS

Referring firstly to FIG. 1, there is shown a schematic side view of adownhole tool assembly in accordance with an embodiment of the presentinvention, indicated generally by reference numeral 10, shown located ina well 12.

The downhole tool assembly comprises tubing such as production tubing 14extending to surface and located in a borehole 16 of the well 12, whichhas been lined with lining tubing (not shown) in a fashion known in theart. The downhole tool assembly includes a downhole pump assembly 18coupled to the production tubing 14 and located in the borehole 16 in aregion 20 of a well fluid producing formation 22. The formation 22 hasbeen perforated to produce perforations 24 extending into the formationto allow well fluid to flow into the borehole 16, as shown in FIG. 1.

The pump assembly 18 generally includes a turbine 26 coupled to a pump28, for driving the pump 28 to recover well fluid from the formation 22.In more detail, and viewing FIG. 1 from top to bottom, the downhole pumpassembly 18, in particular the turbine 26, is coupled to the productiontubing 14 by dedicated turbine drive fluid tubing 30. The turbine drivefluid tubing 30 is provided within the production tubing 14 and extendsto surface. Well fluid return tubing 32 is also coupled to theproduction tubing 14, both tubings 30 and 32 banded at 34 to theproduction tubing 14. The well fluid return tubing 32 may be providedwithin the production tubing 14 and extend to surface or may communicatewith the production tubing 14 so as to provide a fluid production pathto surface. Both the tubings 30 and 32 may comprise coil tubing, forease of installation.

The production tubing 14 extends within the casing/lining (not shown) tosurface, in a known fashion, to an offshore or onshore oil/gas rig. Amotor/pump set (not shown) at surface delivers turbine drive fluid(typically seawater in this embodiment) down the production tubing 14and through the turbine drive fluid tubing 30 to the turbine 26, asindicated by the arrow A in FIG. 1. The turbine 26 includes a turbineunit 36 and a turbine discharge 38, and the turbine drive fluid passesdown through the turbine unit 36, to drive the turbine, as will bedescribed with reference to FIGS. 3 to 13. The spent drive fluid isdischarged from the turbine unit 36 at the turbine discharge 38, andflows into a generally annular chamber 40 defined between the pumpassembly 18 and the walls of the borehole 16, the fluid flowing in thedirection of the arrow B shown in FIG. 1.

The turbine drive fluid may comprise seawater, but recovered well fluidmay alternatively be used on its own or in combination with anotherdrive fluid, such as seawater. In particular, well fluid recovered tosurface may be pumped back down through the turbine drive fluid tubing30 for driving the turbine. The well fluid may be separated at surfaceinto hydrocarbons (oils, gases and/or condensates) and water, and therecovered water or oil re-injected and used as the drive fluid. In otheralternatives, the turbine may be steam driven or gas driven, forexample, using air, Nitrogen or a Nitrogen foam.

The pump 28 is coupled to the turbine by a drive shaft (not shown)extending through the turbine discharge 38 and includes a pump unit 42having a pump discharge 44 forming an outlet of the pump 28. The pumpunit 42 comprises a typical pump unit such as those employed in current.ESP assemblies, and includes a pump inlet 21 for drawing fluid into thepump 28, for recovering well fluid to surface. The pump inlet 21 isisolated from the pump outlet in the pump discharge 44, and thereforefrom the turbine discharge 38, by isolation means in the form of apacker 46. The packer 46 receives, locates and seals the pump 28 in theborehole 16 casing. In this fashion, the pump unit 28 acts mainly todraw well fluid from the formation 22, and does not have to carry outadditional work to pump discharged turbine drive fluid through the pump.

When the turbine 26 is activated to drive the pump 28, well fluid 48 isdrawn into and through the pump in the direction of the arrow C,discharging from the pump discharge 44 in the direction D, into thechamber 40. The well fluid 48 mixes with discharged turbine drive fluidin the chamber 40, and is pumped up through the well fluid return tube32 to surface, in the direction of the arrow E. An upper isolation meansin the form of a packer 50 seals the tubing 30 and 32, to direct themixed well fluid and turbine drive fluid into the return tubing 32 andthus to surface, where the well fluid is separated from the turbinedrive fluid. As discussed, at least part of the separated turbine drivefluid may be recycled downhole for further driving the turbine 26.

The pump 28 is sized for the flow rate to be drawn from the formation 22and the pressure head requirement at the depth of the pump assembly 18.Also, the absolute pressure of the drive fluid at the inlet 52 of theturbine 36 is set such that the differential pressure extracted by theturbine 36 from the drive fluid will cause the exhaust pressure from theturbine 36 to be roughly equivalent to the annulus pressure at the depthof the pump assembly 18. Each of the turbine 26 and pump 28 includesrespective thrust bearings (not shown), such that axial loads in theturbine and pump are carried by respective self-contained bearings.

Turning now to FIG. 2, there is shown a downhole tool assembly 10 a. Theassembly 10 a is similar to the assembly 10 of FIG. 1, and likecomponents share the same reference numerals with the addition of theletter “a”. For brevity, only the differences between the assembly 10 aand the assembly 10 will be described.

The turbine 26 a of the downhole pump assembly 18 a is coupled directlyto production tubing 14 a such that turbine drive fluid is directedthrough the production tubing 14 a into the turbine unit 36 a in thedirection of the arrow F, before discharging from the turbine discharge38 a in the direction of the arrow G. In this fashion, reservoir fluidflowing through the pump unit 42 a in the direction C, and dischargingfrom the pump discharge 44 a in the direction D, mixes with thedischarged turbine drive fluid in the borehole annulus 54, and isreturned to surface up the annulus 54. This avoids the costs associatedwith acquiring and installing the coiled tubing of the turbine drivefluid and well fluid tubings 30, 32 of the assembly 10.

Turning now to FIG. 2A, there is shown a downhole tool assembly 10 b.The assembly 10 b is similar to the assemblies 10 and 10 a of FIGS. 1and 2, and like components share the same reference numerals with theletter “b”. For brevity, only the differences between the assembly 10 band the assemblies 10 and 10 a will be described.

The assembly 10 b is similar to the assembly 10 a of FIG. 2A in that thedownhole pump assembly 18 b is coupled directly to production tubing 14b such that turbine drive fluid is directed through the Productiontubing 14 b into the turbine unit 36 b, as shown by the arrow H.However, the pump assembly 18 b also includes discharge means in theform of a discharge tube 56, which extends from the pump unit 42 b. Theturbine drive fluid flowing down through the turbine 36 b passes alsothrough the pump unit 42 b, and the tube 56 isolates the drive fluidfrom the pump inlet 21 b.

Isolation means in the form of a lower packer 58 isolates an outlet 60of the discharge tube 56, which essentially defines an outlet of theturbine 36 b. The region 20 b of the production formation extends over alength of the borehole 16 b and fluid flows from upper perforations 24 binto the pump inlet 21 b in the fashion described above. The fluid thenexits a pump discharge 44 b which is provided around or with the turbine36 b, and flows up the annulus 54 b to surface, in the direction of thearrow I.

Spent turbine drive fluid flowing down through the discharge tube 56exits the outlet 60 and is injected into the formation 20 b throughlower perforations 62. Thus well fluids drawn from the formation 20 bare replaced by injected, spent turbine drive fluid, as shown by thearrows J in the Figure. This spent fluid is prevented from flowing backup through the borehole 16 b by the packer 58, and maintains theformation pressure at an acceptable level for well fluids to continue tobe withdrawn. Whilst FIG. 2A is a schematic view of the borehole 16 band pump assembly 18 b, it will be understood that the outlet 60 of thedischarge tube 56 is spaced at some distance from the pump assembly 18 band the perforations 24 b. This distance may be hundreds or thousands offeet, such that the spent turbine drive fluid is exhausted from the pumpassembly 18 b in a different zone from that where oil is being extracted(the region where the perforations 24 b are located). This obviates therequirement to separately inject fluid into the well to maintainformation pressure, as may be required with the embodiments of FIGS. 1and 2. A pressure drop occurs in pumping the spent turbine drive fluiddown the discharge tube 56 to the outlet 60 and up the annulus aroundthe discharge tube and the pressure differential across the turbine maytherefore be relatively large.

It will also be understood that the assemblies of FIGS. 2 and 2A may bedriven using recovered well fluids as described in relation to FIG. 1.

Turning now to FIG. 3, the turbine 36 is shown in more detail. Whilstthe downhole pump assemblies 18 and 18 a of FIGS. 1, 2 and 2A include asingle turbine unit 36, it will be appreciated that any desired number,for example two or more, turbine units may be provided. Accordingly, aswill be described below, FIG. 4B illustrates the connection of theturbine unit 36 to a second such unit 37.

The following description applies to the turbines 26, 26 a and 26 b ofFIGS. 1 to 2A. However, for clarity, only the turbine 26 is hereindescribed. As shown in FIG. 3, a top connecting sub 103 is coupled tothe turbine unit 36, which comprises an outer casing 111 in which isfixedly mounted a stator 112 having a generally lozenge-section outerprofile 113 defining with the outer casing 111 two diametrically opposedgenerally semi-annular drive fluid supply passages 114 therebetween. Atthe clockwise end 115 of each passage 114 is provided a conduit 116providing a drive fluid supply nozzle 117 directed generallytangentially of a cylindrical profile chamber 118 defined by the stator112 inside which is disposed a rotor 119.

The rotor 119 is mounted rotatably via suitable bushings and bearings(not shown) at end portions 120,121 which project outwardly of each end122,123 of the stator 112. As shown in FIGS. 5 to 7, the rotor 119comprises a tubular central member 124 which is closed at the upper endportion 120 and, between the end portions 120,121, has a series ofspaced apart radially inwardly slotted 125 flanges 126 in which arefixedly mounted cylindrical tubes 127 (see FIGS. 6 & 7) extendinglongitudinally of the rotor. FIG. 6 is a transverse section through aflange 126 which supports the base and sides of the tubes 127 thereat.FIG. 7 is a transverse section of the rotor 119 between successiveflanges 126 and shows a series of angularly spaced exhaust apertures 128extending radially inwardly through the tubular central member 124 to acentral axial drive fluid exhaust passage 129. Between the flanges 126,the tubes 127 are cut-away to provide angularly spaced apart series ofsemi-circular channel section buckets 130 forming, in effect, a seriesof turbine wheels 130 a interspersed by supporting flanges 126. Thebuckets 130 are oriented so that their concave inner drive fluidreceiving faces 131 face anti-clockwise and rearwardly of the normalclockwise direction of rotation of the turbine rotor 119 in use of theturbine. The buckets 130 are disposed substantially clear of the centraltubular member 124 so that drive fluid received thereby can flow freelyout of the buckets 130 and eventually out of the exhaust apertures 128.With the rotor 119 being enclosed by the stator 112 it will beappreciated that in addition to the “impulse” driving force applied to abucket 130 directly opposite a nozzle 117 by a jet of drive fluidemerging therefrom, other buckets will also receive a “drag” drivingforce from the rotating flow of drive fluid around the interior of thechamber 118 before it is exhausted via the exhaust apertures 128 andpassage 129.

As shown in the alternative embodiment of FIG. 4B, which includes twoturbine units 36, 37, the rotor 119 of the upper turbine 36 is drivinglyconnected via a hexagonal (or similar) coupling 132 to the rotor of thelower turbine 37, which is substantially similar to the upper turbine36. In a still further alternative embodiment, the lower turbine 37 maybe in turn drivingly connected via a single or by upper and lower gearboxes (not shown) and suitable couplings to the pump 28. As shown inFIG. 8 the or each gear box may be of epicyclic type with a driven sunwheel 136, a fixed annulus 137, and four planet wheels 138 mounted in acage 139 which provides an output drive in the same direction as thedirection of rotation of the driven sun wheel 136.

In use of the turbine 36, the motive fluid enters the top sub 103 andpasses down into the semi-annular supply passages 114 of the upperturbine 36 between the outer casing 111 and stator 112 thereof, whenceit is jetted via the nozzles 117 into the chamber 118 in which the rotor119 is mounted, so as to impact in the buckets 130 thereof. The motivefluid is exhausted out of the chamber 118 via the exhaust apertures 128down the central exhaust passage 129 inside the central rotor member124, until it reaches the lower end 124 a thereof engaged in thehexagonal coupling 32 (where two turbine units 36, 37 are provided),drivingly connecting it to the closed upper end 124 b of the rotor 119of the lower turbine 37. Of course, where the turbine 26 includes onlythe single turbine unit 36, the drive fluid is exhausted from theturbine discharge 38, as shown in FIG. 1. The fluid then passes radiallyoutwards out of apertures 132 a provided in the hexagonal coupling 132of the lower turbine and then passes along into the semi-annular supplypassages 114 of the lower turbine 37 between the outer casing 111 andstator 112 thereof to drive the lower turbine 37 in the same way as theupper turbine 36. It will be appreciated that the lower turbine iseffectively driven in series with the upper turbine. This is thoughquite effective and efficient given the highly efficient “parallel”driving within each of the upper and lower turbines. The drilling motivefluid exhausted from the lower turbine then passes along centralpassages extending through the interior of the gear boxes (whereprovided), discharging at the discharge 0.38.

With a single turbine unit as shown in the drawings suitable for use ina 3.125 inch (8 cm) diameter bottom hole assembly and a drive fluidsupply pressure of 70 kg/cm² there may be obtained an output torque ofthe order of 2.5 m.kg at 6000 rpm. With a 3:1 ratio gearing down therecan then be obtained an output torque of the order of 9 m.kg at 2000rpm. With a system as illustrated there can be obtained an output torqueof the order of 25 m.kg at 600 rpm which is comparable with theperformance of a similarly sized conventional Moineau motor orconventional downhole turbine having a diameter of 4¾″ (12 cm) and 50 ft(15.24 m) length.

It will be appreciated that various modifications may be made to theabove described turbine. Thus for example the profiles of the buckets130 and their orientation, and the configuration and orientation of thenozzles 117, may all be modified so as to improve the efficiency of theturbine.

The turbine 236 shown in FIGS. 9-13 is generally similar to that ofFIGS. 3-8, comprising an outer casing 141 in which is fixedly mounted astator 142 having a generally lozenge-section outer profile 143 definingwith the outer casing 141 four angularly distributed generallysegment-shaped drive fluid supply passages 144 therebetween. At theclockwise end 145 of each passage 144 is provided a drive fluid supplyconduit 146 providing a drive fluid supply nozzle 147 directed generallytangentially of a cylindrical profile chamber 148 defined by the stator142 inside which is disposed a rotor 149.

The rotor 149 is mounted rotatably via suitable bushings and bearings150, 151 at the end portions 152 a, 152 b which project outwardly ofeach end 153 a, 153 b of the stator 142. As shown in FIGS. 10, 11 and 12the rotor 149 comprises an elongate tubular central member 154 which hasa series of axially spaced apart radially inwardly slotted 155 flanges156 in which are fixedly mounted four axially spaced apart sets ofcylindrical tube profile or aerodynamically/hydrodynamically shapedturbine blades 157 providing an array of four turbine wheel blade arrays158A-D extending longitudinally along the central rotational axis of therotor 149. FIG. 10 is a transverse section through a turbine wheel bladearray 158A and shows four nozzles 147 for directing jets of drive fluidinto the blades 157 and a series of six angularly spaced apart exhaustapertures 159′ extending radially inwardly through the tubular centralmember 154 to an inner drive fluid exhaust passage 159. Inside thetubular central member 154 is provided a spindle member 160 mounting aseries of annular sealing members 161A-C for isolating lengths of innerdrive fluid exhaust passage 159′ A-C, from each other. A further lengthof inner drive fluid exhaust passage 159′D is isolated from thepreceding length 159′C. by an integrally formed end wall 162.

Between the opposed flanges 156′, 156″ of each pair of successiveturbine wheel blade arrays 158A-D, the stator 142 is provided withrelatively large apertures 163 which together with apertures 164 in thetubular central member 154 provide drive fluid return flow passages 165for conducting drive fluid exhausted from the exhaust apertures 159 ofan upstream turbine wheel blade array 158A into the respective innerdrive fluid exhaust passage 159′, to the drive fluid supply passage 144of a turbine wheel blade array 158B immediately downstream thereof forserial interconnection of said turbine wheel blade arrays 158A, 158B. Asshown in FIG. 11, the apertures 164 in the tubular central member 154are orientated generally tangentially in order to improve fluid flowefficiency.

As may be seen from the drawings, the drive fluid supply conduits 146are in the form of relatively large slots having an axial extent almostequal to that of the turbine blades 157 so that the fluid flow capacityand power of each turbine wheel blade array 158A etc is actually similarto that of the or each of the turbine units 36, 37, with its series of12 turbine wheel blade arrays connected in parallel (as illustrated inFIG. 5) of the above described turbine embodiment. In order to isolatethe drive fluid supply passages 144 of successive turbine wheel bladearrays 158A, 158B etc from each other, the flanges 156 supporting theturbine blades 157 are provided with low-friction labyrinth seals 166around their circumference.

As will be apparent from FIG. 9, the close and compact coupling andarrangement of the four turbine wheel blade arrays 158A-D, requires amuch smaller amount of bearings and seals thereby considerably reducingfrictional losses as compared with the type of arrangement illustratedin FIGS. 3-5, as well as considerably reduced length, thereby providinga much higher torque and power output for a given length and size ofturbine, as compared with previously known turbines.

In other respects the turbine of FIGS. 9-13 is generally similar to thatof FIGS. 3-8. Thus the turbine blades 157 form concave buckets 167oriented so that their concave inner drive fluid receiving faces 168face anti-clockwise and rearwardly of the normal clockwise direction ofrotation of the turbine rotor 149 in use of the turbine drive and fluidreceived thereby can flow freely out of the buckets 167 and eventuallyout of the exhaust apertures 159.

In use of the apparatus, the motive/drive fluid enters the top sub 103and passes down into the supply passage 144 of the first turbine wheelblade array 158A between the outer casing 141 and stator 142 thereof,whence it is jetted via the nozzles 147 into the chamber 148 in whichthe rotor 149 is mounted so as to impact in the buckets 167 thereof. Themotive fluid is exhausted out of the chamber 148 via the exhaustapertures 159 into the central exhaust passage 159′ inside the centraltubular member 154 whereupon it is returned radially outwardly via thedrive fluid return flow passage 165 to the drive fluid supply passage144 of the next turbine wheel blade array 158B, whereupon the process isrepeated.

With a four stage integrated turbine unit as shown in FIGS. 9 to 13 foruse in a 3.125 inch (8 cm) diameter bottom hole assembly and a drivefluid mass flow of 110 US gallons per minute (416 litres per minute) anda supply pressure of 1000 psi (70 kg/cm²) there may be obtained anoutput of 8200 rpm and 17.4 ft-lbs (2.4 m.kg). With a 12:1 ratio gearingdown there can be obtained an output torque of 208.4 ft-lbs (28.8 m.kg)at 683 rpm, which is comparable with the performance of a similarlydiametrically sized conventional Moineau motor but of twice the lengthof a conventional downhole turbine of greater diameter and more thanfour times the length.

Various modifications may be made to the foregoing within the scope ofthe present invention.

Either one or both of the turbine drive fluid delivery tubing and/orwell fluid return tubing may extend to surface.

1. A downhole pump assembly comprising a turbine and a pump, the turbinebeing coupled to the pump for driving the pump, and wherein the turbineis a radial flow turbine, wherein the turbine includes a fluid outletisolated from a fluid outlet of the pump, wherein the turbine fluidoutlet is spaced from the pump for discharging turbine drive fluid at alocation spaced from the pump, and wherein the turbine fluid outlet islocated, in use, further downhole than the pump, fluid outlet; whereinthe pump is fluidly isolated from the turbine by a packer, and whereinthe pump is adapted to be located in the packer such that the packerseals an annulus defined between the pump and a borehole in which theassembly is located.
 2. The downhole pump assembly as claimed in claim1, wherein, in use, the pump is disposed downhole of the turbine.
 3. Thedownhole pump assembly as claimed in claim 1, wherein the pump isadapted to pump well fluid uphole so as to recover well fluid tosurface.
 4. The downhole pump assembly as claimed in claim 3, wherein atleast part of the pump is isolated from at least part of the turbine. 5.The downhole pump assembly as claimed in claim 3, wherein the pumpincludes a pump fluid inlet and a pump fluid outlet, and wherein thepump inlet is fluidly isolated from at least part of the turbine.
 6. Thedownhole pump assembly as claimed in claim 5, wherein the pump fluidinlet is fluidly isolated from a fluid outlet of the turbine.
 7. Thedownhole pump assembly as claimed in claim 3, wherein the turbineincludes a turbine fluid outlet isolated from a fluid outlet of thepump.
 8. The downhole pump assembly as claimed in claim 7, wherein theturbine fluid outlet is spaced from the pump for discharging turbinedrive fluid at a location spaced from the pump.
 9. A downhole pumpassembly comprising a turbine and a pump, the turbine being coupled tothe pump for driving the pump, and wherein the turbine is a radial flowturbine, wherein the pump is fluidly isolated from the turbine by apacker, and wherein the pump is adapted to be located in the packer suchthat the packer seals an annulus defined between the pump and a boreholein which the assembly is located, wherein the turbine and pump includeoutlets disposed upstream of the packer.
 10. The downhole pump assemblyas claimed in claim 9, further comprising discharge tubing coupled tothe pump assembly and defining an outlet forming a fluid outlet of theturbine.
 11. The downhole pump assembly as claimed in claim 9, whereinthe turbine is directly coupled to the pump.
 12. The downhole pumpassembly as claimed in claim 9, wherein the turbine is coupled to thepump for driving the pump to recover well fluid to surface, the downholepump assembly further comprising a gear unit between the turbine and thepump.
 13. The downhole pump assembly as claimed in claim 9, comprisingdelivery tubing for supplying drive fluid to the turbine and returntubing for returning well fluid to surface.
 14. The downhole pumpassembly as claimed in claim 13, wherein the delivery and return tubingcomprise coiled tubing.
 15. The downhole pump assembly as claimed inclaim 13, wherein the delivery and return tubing is sealed by isolationmeans to constrain return flow to surface to be directed through thereturn tubing.
 16. The downhole pump assembly as claimed in claim 9,wherein the downhole pump assembly is adapted to be coupled directly todownhole tubing for supplying turbine drive fluid to the downhole pumpassembly, and wherein the downhole pump assembly is adapted to recoverwell fluid through an annulus defined between a borehole in which thedownhole pump assembly is located and the downhole pump assembly. 17.The downhole pump assembly as claimed in claim 13, wherein the deliverytubing extends to surface.
 18. A downhole pump assembly comprising aturbine and a pump, the turbine being coupled to the pump for drivingthe pump, and wherein the turbine is a radial flow turbine, wherein thedownhole pump assembly is adapted to be coupled directly to downholetubing for supplying turbine drive fluid to the assembly, and whereinthe assembly is adapted to recover well fluid through an annulus definedbetween a borehole in which the assembly is located and the assembly,further comprising discharge tubing extending through the turbine andthe pump to a discharge location spaced from the assembly.
 19. Adownhole pump assembly comprising a turbine and a pump, the turbinebeing coupled to the pump for driving the pump, and wherein the turbineis a radial flow turbine, wherein the turbine, in use, drive fluidentering a chamber from a supply passage via nozzle means impactsturbine blade means, the drive fluid exhausting from the chamber viaoutlet apertures angularly spaced from the nozzle means in a downstreamdirection and into exhaust passages.
 20. The downhole pump assembly asclaimed in claim 19, wherein the turbine blade means are, in use, drivenby radial flow of the drive fluid, and wherein the pump is adapted topump well fluid uphole so as to recover well fluid to surface, whereinthe rotational velocity of the turbine is adjustable to balance therotational velocity of the turbine with a rotational velocity of thepump.
 21. A downhole pump assembly comprising a turbine and a pump, theturbine being coupled to the pump for driving the pump, and wherein theturbine is a radial flow turbine, wherein the turbine comprises atubular casing enclosing a chamber having rotatably mounted therein arotor comprising enclosing a chamber having rotatably mounted therein arotor comprising at least one turbine wheel blade array with an annulararray of angularly distributed blades orientated with drive fluidreceiving faces thereof facing generally rearwardly of a forwarddirection of rotation of the rotor, and a generally axially extendinginner drive fluid passage generally radially inwardly of said rotor,said casing having a generally axially extending outer drive fluidpassage, one of said inner and outer drive fluid passages constituting adrive fluid supply passage and being provided with at least one outletnozzle formed and arranged for directing at least one jet of drive fluidonto said blade drive fluid receiving faces of said at least one bladearray as said blades traverse said nozzle for imparting rotary drive tosaid rotor, the other constituting a drive fluid exhaust passage andbeing provided with at least one exhaust aperture for exhausting drivefluid from said at least one turbine wheel blade array.
 22. A downholepump assembly comprising a turbine and a pump, the turbine being coupledto the pump for driving the pump, and wherein the turbine is a radialflow turbine, wherein the turbine comprises a tubular casing enclosing achamber having rotatably mounted therein a rotor having at least twoturbine wheel blade arrays each with an annular array of angularlydistributed blades orientated with drive fluid receiving faces thereoffacing generally rearwardly of a forward direction of rotation of therotor, and a generally axially extending inner drive fluid passagegenerally radially inwardly of each said turbine wheel blade array, saidcasing having a respective generally axially extending outer drive fluidpassage associated with each said turbine wheel blade array, one of saidinner and outer drive fluid passages constituting a drive fluid supplypassage and being provided with at least one outlet nozzle formed andarranged for directing at least one jet of drive fluid onto said bladedrive fluid receiving faces as said blades traverse said at least onenozzle for imparting rotary drive to said rotor, the other constitutinga drive fluid exhaust passage and being provided with at least oneexhaust aperture for exhausting drive fluid from said turbine wheelblade arrays, neighbouring turbine wheel blade arrays being axiallyspaced apart from each other and provided with drive fluid return flowpassages therebetween connecting the exhaust passage of an upstreamturbine wheel blade array to the supply passage of a downstream turbinewheel blade array for serial interconnection of said turbine wheel bladearrays.
 23. A downhole pump assembly comprising a turbine and a pump,the turbine being coupled to the pump for driving the pump, and whereinthe turbine is a radial flow turbine, wherein the rotational velocity ofthe turbine is adjustable to balance the rotational velocity of theturbine with that of the pump, wherein the size of a nozzle of theturbine is adjustable to vary the rotational velocity of the turbine, tobalance the rotational velocity of the turbine to that of the pump. 24.The downhole pump assembly as claimed in claim 23, wherein the pump isadapted to pump well fluid uphole so as to recover well fluid tosurface, and wherein the turbine is adapted to be driven at least inpart by recovered well fluid.
 25. An assembly as claimed in claim 24,wherein the turbine is adapted to be driven at least in part by waterseparated from the recovered well fluid.
 26. An assembly as claimed inclaim 24, wherein the turbine is adapted to be driven at least in partby oil separated from the recovered well fluid.
 27. A downhole pumpassembly comprising a turbine and a pump, the turbine being coupled tothe pump for driving the pump, and wherein the turbine is a radial flowturbine, wherein the turbine includes a fluid outlet isolated from afluid outlet of the pump, wherein the turbine fluid outlet is spacedfrom the pump for discharging turbine drive fluid at a location spacedfrom the pump, and wherein the turbine fluid outlet is located, in use,further downhole than the pump fluid outlet; wherein the pump is adaptedto pump well fluid uphole so as to recover well fluid to surface; andwherein a fluid outlet of the pump is disposed in fluid communicationwith a fluid outlet of the turbine.
 28. A method of recovering wellfluids, the method comprising: coupling a turbine to a pump to form adownhole pump assembly, the downhole assembly comprising the turbine andthe pump, wherein the turbine is a radial flow turbine, wherein theturbine includes a fluid outlet isolated from a fluid outlet of thepump, wherein the turbine fluid outlet is spaced from the pump fordischarging turbine drive fluid at a location spaced from the pump, andwherein the turbine fluid outlet is located, in use, further downholethan the pump fluid outlet, and the pump being adapted to pump wellfluid uphole so as to recover well fluid to surface; and coupling thedownhole pump assembly to downhole tubing; running the downhole tubingand downhole pump assembly into a borehole of a well and locating thepump assembly in a region of a well fluid producing formation; andsupplying drive fluid downhole to drive the turbine, to in turn drivethe pump and recover well fluid from the borehole, the method furthercomprising: coupling the turbine directly to production tubing andsupplying drive fluid through the production tubing to drive theturbine, and recovering well fluid through an annulus defined betweenthe downhole pump assembly and the borehole.
 29. A method of recoveringwell fluids, the method comprising: coupling a turbine to a pump to forma downhole pump assembly, the downhole assembly comprising the turbineand the pump, wherein the turbine is a radial flow turbine, wherein theturbine includes a fluid outlet isolated from a fluid outlet of thepump, wherein the turbine fluid outlet is spaced from the pump fordischarging turbine drive fluid at a location spaced from the pump, andwherein the turbine fluid outlet is located, in use, further downholethan the pump fluid outlet, and the pump is adapted to pump well fluiduphole so as to recover well fluid to surface; coupling the downholepump assembly to downhole tubing; running the downhole tubing anddownhole pump assembly into a borehole of a well and locating the pumpassembly in a region of a well fluid producing formation; supplyingdrive fluid downhole to drive the turbine, to in turn drive the pump andrecover well fluid from the borehole; and mixing well fluid with turbinedrive fluid discharged from the turbine in the region of an outlet ofthe pump and returning the well fluid to surface.
 30. The method asclaimed in claim 29, further comprising injecting spent turbine drivefluid into the formation.
 31. The method as claimed in claim 30comprising coupling discharge means to the downhole pump assemblydefining a turbine outlet and isolating the turbine outlet from thepump, to inject spent drive fluid into the formation.
 32. The method asclaimed in claim 30, comprising injecting spent turbine drive fluid intothe formation at a location spaced from the downhole pump assembly. 33.The method as claimed in claim 30, comprising supplying drive fluid atleast partly comprising recovered well fluid to the turbine to drive theturbine.
 34. The method as claimed in claim 33, comprising supplyingdrive fluid at least partly comprising recovered water.
 35. The methodas claimed in claim 33, comprising supplying drive fluid at least partlycomprising recovered oil.
 36. The method as claimed in claim 33,comprising separating recovered well fluid into at least water and oilcomponents and supplying the separated water to the turbine to drive theturbine.
 37. The method as claimed in claim 29, comprising supplyingdrive fluid at least partly comprising a gas to the turbine to drive theturbine.
 38. The method as claimed in claim 29, comprising supplyingdrive fluid at least partly comprising steam to the turbine to drive theturbine.
 39. A method of recovering well fluids, the method comprising:coupling a turbine to a pump to form a downhole pump assembly, thedownhole assembly comprising the turbine and the pump, wherein theturbine is a radial flow turbine, wherein the turbine includes a fluidoutlet isolated from a fluid outlet of the pump, wherein the turbinefluid outlet is spaced from the pump for discharging turbine drive fluidat a location spaced from the pump and wherein the turbine fluid outletis located, in use, further downhole than the pump fluid outlet, and thepump is adapted to pump well fluid uphole so as to recover well fluid tosurface; coupling the downhole pump assembly to downhole tubing; runningthe downhole tubing and downhole pump assembly into a borehole of a welland locating the pump assembly in a region of a well fluid producingformation; supplying drive fluid downhole to drive the turbine, to inturn drive the pump and recover well fluid from the borehole; andbalancing the operational velocity of the turbine to that of the pump.40. A method of recovering well fluids, the method comprising: couplinga turbine to a pump to form a downhole pump assembly, the downholeassembly comprising the turbine and the pump, the turbine being coupledto the pump for driving the pump, and wherein the turbine is a radialflow turbine, wherein the turbine includes a fluid outlet isolated froma fluid outlet of the pump, wherein the turbine fluid outlet is spacedfrom the pump for discharging turbine drive fluid at a location spacedfrom the pump, and wherein the turbine fluid outlet is located, in use,further downhole than the pump fluid outlet; and coupling the downholepump assembly to downhole tubing; running the downhole tubing anddownhole pump assembly into a borehole of a well and locating the pumpassembly in a region of a well fluid producing formation; and supplyingdrive fluid downhole to drive the turbine, to in turn drive the pump andrecover well fluid from the borehole; further comprising balancing theoperational velocity of the turbine to that of the pump; and adjustingthe size of an outlet nozzle of the turbine formed and arranged fordirecting at least one jet of drive fluid onto a turbine blade array ofthe turbine to vary the flow velocity of fluid through the turbine.